1. Field of the Invention
The invention relates to a high-strength cover glass that is chemically tempered or temperable by ion exchange and exhibits excellent scratch behavior. The glass may be used as a protective cover glass in electronic devices, such as smart phones, tablet PCs, navigation devices, etc.
2. Description of Related Art
Smart phones, tablet PCs, navigation devices, etc. are generally operated via touch screens nowadays. For protecting the display and the sensor, thin ion-exchanged (chemically tempered) glasses can be used. Chemical tempering of the glass is achieved by exchanging small alkali ions (e.g. Na+) for larger homologs (e.g. K+). In this manner, a stress profile is generated in the glass.
After the ion exchange there will be a compressive stress zone in the region near the surface of the glass and a tensile stress zone in the inner region. The compressive stress zone in the glass surface obtained by the ion exchange is accompanied by a strong increase in bending strength of the glasses, which can be demonstrated impressively by mechanical fracture tests (e.g. 4 point bending test, falling ball experiments, double ring test). To this end, it would be beneficial to achieve sufficiently high compressive stresses of more than 700 MPa in the surface and exchange depths of more than 25 μm by the ion exchange. Below, the designation CS will be used for the Compressive Stress in the surface of the glass, and the designation DoL (Depth of Layer) will be used for the exchange depth of alkali ions. The CS and DoL values can be measured by a photoelastic method. Suitable for this purpose, for example, is the FSM 6000 measuring device of Luceo Co., Ltd., Japan.
Glasses of the system of alkali alumosilicates have turned out to be particularly well ion-exchangeable and resistant. Currently, various glasses of this type are used as cover glasses to protect touch displays of electronic devices. The composition of the glasses has a strong influence on the values of compressive stress CS in the surface of the glass and on the exchange depth DoL (Depth of Layer) resulting from the ion exchange.